CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Post-Weld Heat Treatment on Hydrogen-Induced Delamination in Stainless Steel Overlay Layers
Literature Overview
This paper, published in Shanghai Metals in 1995 by researchers from Shanghai Jiao Tong University and East China University of Science and Technology, addresses one of the most insidious failure modes in clad plate and weld overlay fabrication: hydrogen-induced delamination (HID) at the interface between the stainless steel overlay layer and the carbon steel base metal. The study systematically investigates how post-weld heat treatment (PWHT) parameters influence the susceptibility of the overlay interface to hydrogen embrittlement, providing critical guidance for fabrication engineers working on austenitic stainless steel cladding over low-carbon or low-alloy steel substrates.
Core Technical Findings
The research identifies that residual hydrogen trapped during the welding process is the primary driving force for delamination at the overlay-base interface. When the overlay is deposited onto a carbon steel substrate, the hydrogen generated by the arc process can diffuse into the weld zone and accumulate at the interface. The study demonstrates that PWHT is not merely a stress-relief operation but plays a decisive role in hydrogen diffusion and redistribution.
Key PWHT Parameters Investigated
| Parameter | Typical Range Studied | Effect on HID Susceptibility |
|---|---|---|
| PWHT Temperature | 250-350°C | Lower temperatures favor hydrogen escape; higher temperatures risk sensitization |
| Dwell Time | 1-4 hours | Insufficient dwell allows residual hydrogen retention |
| Cooling Rate | Controlled slow cooling | Rapid cooling traps hydrogen in the microstructure |
| Pre-heat Temperature | 100-200°C | Higher pre-heat reduces hydrogen ingress during welding |
The study concludes that a PWHT cycle at 300-320°C with a dwell time of at least 2 hours per 25 mm of thickness provides optimal hydrogen diffusion out of the overlay interface region. Below 250°C, the hydrogen diffusion coefficient is insufficient to allow complete hydrogen escape within practical dwell times. Above 350°C, however, the austenitic stainless steel overlay layer begins to approach the sensitization temperature range, risking chromium carbide precipitation at grain boundaries and subsequent intergranular corrosion susceptibility.
Mechanism Analysis
The hydrogen-induced delamination mechanism involves three sequential stages. First, atomic hydrogen generated during welding diffuses into the weld metal and the overlay-base interface region. Second, hydrogen atoms accumulate at microstructural traps such as grain boundaries, inclusions, and the diffusion bonding interface. Third, under residual tensile stress or external loading, the accumulated hydrogen reduces the effective cohesive strength of the interface, leading to intergranular or interfacial cracking.
The PWHT facilitates hydrogen diffusion out of the weld zone by increasing the hydrogen diffusion coefficient exponentially with temperature. The Arrhenius-type relationship between hydrogen diffusivity and temperature means that even a modest increase in PWHT temperature can dramatically reduce the hydrogen retention time. However, the temperature window is narrow because austenitic stainless steels such as 304 and 316 begin to form chromium carbides above approximately 425°C, and the risk of sensitization increases progressively in the 450-800°C range.
Engineering Practice Implications
For engineers fabricating clad plate pressure vessels or weld-overlay equipment, this research provides several actionable recommendations. Pre-heating the base metal to 150-200°C before overlay welding significantly reduces the hydrogen concentration in the weld zone by allowing hydrogen to diffuse out during welding rather than being trapped. Using low-hydrogen consumables with hydrogen content below 5 mL/100g of deposited metal is essential. The PWHT should be performed as soon as practically possible after welding, ideally within 4 hours, to prevent hydrogen accumulation.
The study also highlights that the type of overlay process matters. Submerged arc welding (SAW) and electroslag welding (ESW) produce higher hydrogen levels compared to gas tungsten arc welding (GTAW) or plasma transferred arc (PTA) welding, making PWHT even more critical for these processes. Engineers should incorporate hydrogen bake-out cycles into their welding procedure specifications (WPS) and ensure that the PWHT cycle is documented and traceable per NB/T 47014 or ASME IX requirements.
Study Reflections
This 1995 study remains highly relevant to contemporary practice. Despite advances in welding technology and consumable development, hydrogen-induced delamination continues to be a failure mode in the field, particularly in hydrogenation reactors and high-pressure separators where the operating environment itself introduces hydrogen. The fundamental insight that PWHT serves a dual purpose of stress relief and hydrogen removal is something that should be embedded in the training of every welding engineer. The narrow temperature window between effective hydrogen diffusion and sensitization risk demands careful process control and thorough understanding of the metallurgical consequences of each PWHT parameter.